Using pipe in linux using parent and child process - linux

Question

i am trying to implement this linux command using C.
ls -l | cut -b 1
the way i am trying to do it is
calling ls -l in parent process
putting output of ls -l in a file(writing to a file)
calling cut in child process
reading the file (the one written in the parent process)
applying cut to the file
printing the output
this is by far what i have done
/* pipe.c */
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
void main()
{
int filedes[2];
int p;
pid_t pid, pid1;
p=pipe(filedes);
FILE *stream;
char buff[20];
printf("pipe command returns %d, %d ,%d\n",p, filedes[0],filedes[1]);
if(pipe(filedes) == -1) /* Create the pipe */
printf("error pipe");
pid1=fork();
pid=getpid();
switch (pid1) { /* Create a child process */
case -1:
printf("error fork");
case 0: /* Child */
/* Close unused write end */
/* Child can now read from pipe */
if (close(filedes[1]) == -1)
printf("error close");
printf("I am a child process pid %d, and will read from pipe\n",pid);
while (read(filedes[0], &buff, 1) > 0)
write(STDOUT_FILENO, &buff, 1);
write(STDOUT_FILENO, "\n", 1);
close(filedes[0]);
_exit(EXIT_SUCCESS);
break;
default: /* Parent */
/* Close unused read end */
/* Parent can now write to pipe */
if (close(filedes[0]) == -1)
printf("error close");
printf("I am the parent process pid %d, and will write to pipe\n", pid );
stream = fdopen(filedes[1], "w");
strcpy(buff, "This is a test\n");
write(filedes[1], buff, strlen(buff));
char *args[80];
args[0] = "ls";
args[1] = "-l";
args[2] = NULL;
execvp(args[0],args);
int bak, new;
bak = dup(1);
new = open("/home/urwa/abc.txt", O_WRONLY);
dup2(new, 1);
close(new);
close(filedes[1]); /* Reader will see EOF */
wait(NULL); /* Wait for child */
exit(EXIT_SUCCESS);
break;
}
}
this piece of code works perfectly fine. and prints at stand output the test statement. as well as the ls -l output. but the file is empty. what am i doing wrong.
I also tried freopen as follow.. still empty file. :/
FILE *fp;
fp = freopen ("/temp/abc.txt", "a+", stdout);

Answer 1

You didn't called the cut in the child and also the file descriptors are messed up here.
For performing the task you have to close the stdout of parent and make the write end stdout in parent before execvp. In child you have to close stdin of child and make read end as stdin to your child before execvp. In that way your parent's stdout is stdin of your child(creating the pipe b/w two).
int main()
{
int filedes[2];
int p;
pid_t pid = 0, pid1 = 0;
p=pipe(filedes);
FILE *stream;
char buff[20];
char *args[80];
printf("pipe command returns %d, %d ,%d\n",p, filedes[0],filedes[1]);
if(pipe(filedes) == -1) /* Create the pipe */
printf("error pipe");
pid1=fork();
pid=getpid();
switch (pid1) { /* Create a child process */
case -1:
printf("error fork"); break;
case 0: /* Child */
/* Close unused write end */
/* Child can now read from pipe */
if (close(filedes[1]) == -1)
printf("error close");
printf("I am a child process pid %d, and will read from pipe\n",pid);
close(0); //close stdin of child
dup(filedes[0]); //make pipes read end stdin of child
args[0] = "cut";
args[1] = "-b";
args[2] = "1";
args[3] = NULL;
execvp(args[0],args);
break;
default: /* Parent */
/* Close unused read end */
/* Parent can now write to pipe */
if (close(filedes[0]) == -1)
printf("error close");
printf("I am the parent process pid %d, and will write to pipe\n", pid );
close(1); //close stdout
dup(filedes[1]); //make write end of pipe stdout of parent
args[0] = "ls";
args[1] = "-l";
args[2] = NULL;
execvp(args[0],args);
break;
}
}